326 METALS AND THEIR COMBINATIONS. 



All copper salts, except the sulphide, are soluble in ammonia water. Hence, 

 copper cannot be precipitated from ammoniacal solution by any reagent except 

 hydrogen sulphide or alkali sulphides. Copper hydroxide with excess of am- 

 monia water forms the deep-blue soluble compound, Cu(NH 3 ) 4 .(OH) 2 , in which 

 the copper is held in the complex radical or ion, Cu(NH 3 ) 4 ". The ammonio- 

 copper salts are derived from the hydroxide above, and also contain the radical 

 Cu(NH 3 ) 4 ", which is the cause of the blue color. The sulphate has the for- 

 mula Cu(NH 3 ) 4 .SO 4 .H 2 O. 



With excess of alkali cyanides, copper also forms complex double cyanides, 

 from which copper cannot be precipitated by any reagent, not even hydrogen 

 sulphide. 



Poisonous properties and antidotes. The use of copper for culinary vessels 

 Is frequently the cause of poisoning by this metal. A perfectly clean surface of 

 metallic copper is not affected by any of the substances used in the preparation 

 of food, but as the metal is very apt to become covered with a film of oxide 

 when exposed to the air, and as the oxide is easily dissolved by the combined 

 action of water, carbonic or other .acids, such as are found in .vinegar, the juice 

 of fruits, or rancid fats, the use of copper for culinary vessels is always 

 dangerous. Actual adulterations of food with compounds of copper have been 

 detected. 



In cases of poisoning by copper the stomach-pump should be used, vomiting 

 induced, and albumen (white of egg) administered, which forms an insoluble 

 compound with copper. Reduced iron, or a very dilute solution of potassium 

 ferrocyanide, may be of use as antidotes. 



Tests for copper. 

 (Use a 5 per cent, solution of copper sulphate.) 



1. Add to the solution hydrogen sulphide or ammonium sulphide : 

 q, black precipitate of cupric sulphide is formed. (Plate III., 1) : 



CuS0 4 + H 2 S == H 2 SO 4 -f CuS. 



2. Add solution of sodium or potassium hydroxide : a bluish pre- 

 cipitate of cupric hydroxide, Cu(OH) 2 , is formed, which is converted 

 into dark-brown cupric oxide, CuO, by boiling. (See equation 

 above.) (Plate III., 2.) 



3. Add ammonia water : a bluish precipitate of cupric hydroxide 

 is formed which readily dissolves in an excess of the reagent, forming 

 a deep azure-blue solution containing an ammonio-copper compound. 

 (See explanation above.) (Plate III., 3.) 



The delicacy of this test is shown by diluting the solution of the 

 copper salt until its color is no longer visible, and then adding 

 ammonia. 



